With a recent demand for electronic devices and information terminals that are higher in functionality, smaller in size, lighter in weight, and thinner in thickness, there have been developed printed wiring boards that are higher in density, smaller in size, and lighter in weight. Amid such an ongoing development, insulating films used for the printed wiring boards have been required to have higher reliability and workability.
Among printed wiring boards, a flexible printed wiring board is excellent in reliability and is suitable for achieving weight and profile reductions of a printed wiring board because the flexible printed wiring board includes a polyimide film having excellent heat resistance, flame retardancy, electrical insulation reliability, chemical resistance, and mechanical property. Further, such a flexible printed wiring board is so excellent in flexibility as to be folded and mounted in a narrow place. This is an advantage in achieving downsizing of electronic devices.
An insulating film for the flexible printed wiring board has been used, for example, in forms of: (a) a cover lay film obtained by (i) providing an opening(s) on an adhesive-backed cover lay film, which has been obtained by applying an adhesive agent to a polyimide film or the like film, by a stamping process such as punching, (ii) making alignment of the adhesive-backed cover lay film with a circuit through the opening(s), and then (iii) subjecting the adhesive-backed cover lay film to thermocompression bonding onto the circuit by means of a thermal press or the like; (b) a cover coat obtained by (i) directly applying onto a circuit a solution which is prepared by dissolving polyimide resin or the like in an organic solvent, (ii) drying the solvent, and then (iii) curing the solution; and (c) a photosensitive solder resist obtained by (i) directly applying onto a circuit a photosensitive resin composition which is mainly composed of an acid-modified epoxy acrylate, an epoxy resin or the like, (ii) subjecting the photosensitive resin composition to microfabrication of openings by photolithography, and then (iii) curing the photosensitive resin composition applied onto the circuit.
However, the cover lay film, which is a thin film, is difficult to provide an opening(s) with high precision thereon. In addition, the cover lay film is poor in positioning accuracy and laminating workability because alignment of the cover lay film during a laminating process is often made by manually. This results in high cost of production. As for the cover coat, it is difficult to provide an opening(s) with high precision thereon because the cover coat needs to be prepared by selective application of the solution for parts other than the opening(s) of the cover coat by screen printing or the like printing technique, and the cover coat is also less flame resistant. As for the photosensitive solder resist, it is excellent in microfabrication workability because an opening(s) can be formed thereon by photolithography, but it is poor in flexibility such as flexing property and has great cure shrinkage. Due to such shortcomings, the photosensitive solder resist, when provided on a thin circuit board having excellent flexibility such as a printed wiring board, causes a great warpage of the circuit board.
As such an insulating film of the flexible printed wiring board, there have been proposed various insulating films capable of exhibiting various kinds of properties such as workability, flexibility, and flame retardancy.
For example, there have been proposed a resin composition and a cured insulating film each of which contains a novel modified polyimide resin that is excellent in both electrical insulating property and adhesiveness and is also excellent in properties such as heat resistance, flexibility, anti-warpage property, solvent resistance, chemical resistance, and flexing endurance (see, for example, Patent Literature 1).
Further, there has been proposed an insulating film made of a cured product of a thermosetting resin composition that offers a good balance of properties such as printability, tack-free property, matting property, electrical insulation property, and adhesiveness to a coated product (see, for example, Patent Literature 2).
In addition, there has been proposed a photosensitive resin composition that yields an insulating film having a sufficient elongation while ensuring a sufficient flame retardancy without using a halogen-based flame retardant (see, for example, Patent Literature 3).